Integrated circuits are being fabricated in smaller sizes and with electronic circuitry operable at increasingly higher speeds. As the operative speed of electronic circuits increases, it is becoming increasingly more difficult to transmit electrical signals between integrated circuits and boards. For this reason, there is growing interest in developing faster optical interconnect technologies that are operable at high frequencies and compatible with the silicon technology used in integrated circuits.
Crystalline materials are useful in fabricating optical modulators. Crystals have structures existing in a particular orientated arrangement with molecules aligned relative to each other. Upon an applied electric field, the refractive index, absorbance, or other optical properties of crystalline materials may change to a degree commensurate with the applied field. These changes are used to modify the phase, intensity, irradiance, or other properties of a beam of light propagated through a crystalline medium. Referring to FIG. 1A, for example, an optical communication device 10 may comprise a crystal unit 12 sandwiched between first and second electrodes 14, 16, with a power source 15 coupled to the electrodes. Power is applied to the electrodes 14, 16, and a laser 20 is activated to emit a laser beam following the arrow L in FIG. 1A. The laser may be a semiconductor laser, a HeNe laser (632 nm) or the like. The crystal unit 12 is influenced by the applied electric field, and the intensity or phase of the light passing through the crystal unit is changed. As shown in FIG. 1B, the modulator may be coupled to a detector 18 for receiving the light and detecting the variation in the polarization of the light beam to define an optical communication system. Thus, changes in absorption spectra of the crystals with applied fields can be used in the manipulation of light for the presentation, manipulation, and transfer of information, and in the production of efficient optical communication systems.
For use in electro-optic modulators, it has been considered desirable to have good quality crystals available at modest cost. Also, in optimizing the efficacy of a light modulator, it is desirable to keep the absorbance low at the operating wavelength (to minimize waveguide loss), and to maximize the change in optical properties with the applied field. A higher peak differential absorbance (for modulators based on electroabsorption) or peak differential refractive index change (for modulators based on electrorefraction) will translate to a smaller device length sufficient for modulation. The thickness or length of the crystal unit may also affect the degree of modulation. Thus, a higher differential will mean a smaller capacitance and hence higher speeds. A higher differential can also mean lower operating voltages (and hence power). It is desirable in achieving these goals to select appropriate crystalline materials and synthesis procedures that yield monodisperse or nearly monodisperse particles (i.e., nanoparticles possessing a uniform size).
Presently, potassium dihydrogen phosphate (KDP) and aluminum dihydrogen phosphate (ADP) crystals are well known for use in optical modulators. Although such crystals are available in large sizes at relatively low cost, they are soluble in water, fragile, and have large half-wave voltages (that is, large voltages are required for maximum transmission). Lithium tantalate and lithium niobate are more effective and may be used for extremely high frequency products, but their compatability with silicon technology has not yet been satisfactorily demonstrated, and they are also expensive and have a high rejection rate in fabrication. Single-crystal cadmium telluride and gallium arsenide optical materials are well known, but they typically have not been operable at extremely high frequencies. Also, integration with Si-based circuits could be problematic.
Electro-optical modulators also have been described using crystal units comprising organic liquid crystals dispersed in a polymeric material, as shown in U.S. Pat. No. 5,132,814, to Ohkouchi et al, entitled "Liquid Crystal Spatial Light Modulator with Multi-layer Photoconductor," and U.S. Pat. No. 5,011,624, to Yamagishi et al., entitled "Acryate PolymerDispersed Liquid Crystal Material and Device Made Therefrom." Although encapsulated or polymeric-dispersed organic liquid crystal units have advantages, it is difficult to control liquid crystal composites during preparation. Often, liquid crystal droplets have irregular sizes and shapes such that the crystalline material may vary from region to region in its response to the applied electric field. Additionally, organic liquid crystals are generally temperature-sensitive and thermally instable, and the crystalline material may form a clear liquid at increased temperatures. Furthermore, such modulators typically do not operate at the high speeds desired for optical data transmission applications. Nanocrystal-polymer composites have been explored for applications other than optical modulators, that is, for use in light emitting diodes and photoconductors where for most purposes a fast response tme is not essential. See Alivisatos, "Semiconductor Clusters, Nanocrystals, and Quantum Dots," SCIENCE, Vol. 271 (Feb. 16, 1996), at pp. 933-36, which is hereby incorporated by reference.
Accordingly, there remains a need for improved semiconductor crystalline materials useful in electro-optical modulators. Efforts have been made to improve synthesis procedures to obtain nearly monodisperse particles, as disclosed in C. B. Murray, et als., "Synthesis and Characterization of Nearly Monodisperse CdE (E=S, Se, Te), Semiconductor Nanocrystallites," J. AM. CHEM. Soc. Vol. 115, No. 19, pp. 8706-8715 (1993), which is incorporated herein by reference. As noted in Murray, et als., difficulties inhere in the synthesis of II-VI semiconductor crystallites due to polydispersities in size and shape, and due to surface defects and imperfections.
The present invention provides improved crystalline materials exhibiting high peak differential absorbance and refractive index changes that may be used in electro-optical modulators operable at high frequencies and at a broad range of wavelengths. Further features, characteristics, and advantages may appear more fully upon consideration of the detailed description given below.